It is known to use documents having such position identification pattern in combination with a pen having an imaging system, such as an infra red sensitive camera, within it, which is arranged to image a small area of the page close to the pen nib. The pen includes a processor having image processing capabilities and a memory and is triggered by a force sensor in the nib to record images from the camera as the pen is moved across the document. From these images the pen can determine the position of any marks made on the document by the pen. The pen markings can be stored directly as graphic images, which can then be stored and displayed in combination with other markings on the document. In some applications the simple recognition that a mark has been made by the pen on a predefined area of the document can be recorded, and this information used in any suitable way. This allows, for example, forms with check boxes on to be provided and the marking of the check boxes with the pen detected. In further applications the pen markings are analysed using character recognition tools and stored digitally as text. Systems using this technology are available from Anoto AB and described on their website www.Anoto.com.
In order to allow documents to be produced easily with the position identifying pattern on them, it is desirable for the pattern to be suitable for printing on the types of printer that are readily available to a large number of users, such as an ink jet, laser jet or LEP printer. These are digital printers and typically have a resolution of 300, 600 or 1200 dots per inch, and the accuracy with which each dot can be located is variable. Also such printers are generally either monochrome, or, if they are colour printers, have only a small number of ink colours. Therefore, if it is desired to print position coding pattern on a part of a product which has human visible content on it as well, it can be a problem to ensure that the position identifying pattern can be distinguished from the content by the reading device, and that the content remains clearly visible to the human eye, and distinguishable over the content.